1. Field of the Invention
The present invention relates to an etching amount calculating method, a storage medium, and an etching amount calculating apparatus, and in particular to an etching amount calculating method for use in forming concave portions such as trenches and holes on a wafer using a mask film.
2. Description of the Related Art
In a process of manufacturing a semiconductor device, etching in which trenches and holes are formed in a to-be-etched layer of a wafer is carried out using a mask film. In the etching, a to-be-etched layer in a part that is not covered with the mask film is physically/chemically etched by plasma, and in forming a trench, the depth of the trench has to be controlled. Thus, the depth of the trench, i.e. the etching amount has to be calculated during etching, and conventionally, a method using light interference has been widely used as a method of calculating the etching amount.
FIG. 22 is a view useful in explaining light interference during etching.
Referring to FIG. 22, a trench 132 is formed through etching on a wafer W that has a mask film 131 formed on a to-be-etched layer 130. When the laser light L1 is irradiated on the wafer W, reflected light L2 from a surface of the mask film 131, reflected light L3 from a boundary surface between the mask film 131 and the to-be-etched layer 130, and reflected light L4 from a bottom of the trench 132 are produced.
When the reflected lights L2 to L4 are received by a detector, optical path lengths of the reflected lights L2 to L4 differ according to the thickness of the mask film 131 and the depth of the trench 132 as shown in FIG. 22, and hence the phases of the reflected lights L2 to L4 differ on the light-receiving surface of the detector. Therefore, interference light (for example, interference light of the reflected light L2 and the reflected light L4 (hereinafter referred to as “trench interference light”) and interference light of the reflected light L2 and the reflected light L3 (hereinafter referred to as “mask film interference light”)) are produced.
The depth of the trench 132 momentarily changes during etching, and thus the difference in the optical path length of the reflected light L2 and the reflected light L4 also momentarily changes, causing a change in the intensity of the interference light. That is, an interference wave (hereinafter referred to as “trench interference wave”) is produced from the reflected light L2 and the reflected light L4. The period of the interference wave is determined by the rate of change in the depth of the trench 132 (etching rate), and hence the etching rate can be calculated from the period of the interference wave. Further, the etching amount (the depth of the trench 132) can be calculated from the calculated etching rate and an etching time period.
During etching, the mask film 131 is also minutely etched step by step to change its thickness, and hence an interference wave (hereinafter referred to as “mask film interference wave”) is produced from the reflected light L2 and the reflected light L3 as well. Because these interference waves are detected by the same detector, the interference wave detected by the detector consist of a plurality of superposed interference waves having different periods (hereinafter referred to as “superposed interference wave”) (see FIG. 23).
To calculate the depth of the trench 132 (the etching amount of the to-be-etched layer 130) from the superposed interference wave as shown in FIG. 23, the trench interference wave has to be separated from the superposed interference wave.
In the superposed interference wave in FIG. 23, an interference wave with a short period and an interference wave with a long period can be relatively clearly separated. Here, because the rate of change in the depth of the trench 132 is higher than the rate of change in the thickness of the mask film 131 during etching, the period of the trench interference wave is shorter than that of the mask film interference wave. Thus, the interference wave with a short period in the superposed interference wave in FIG. 23 is the trench interference wave, and the period of the trench interference wave can be easily calculated from a time period (“Δt” in the figure) between extreme values of the interference wave with a short period.
In the method in which the time period between extreme values is read from a superposed interference wave, an interference wave with a short period and an interference wave with a long period have to be relatively clearly separated in the superposed interference wave, and the period of the trench interference wave cannot be calculated if it is difficult to separate the interference wave with a short period and the interference wave with a long period in a superposed interference wave. Moreover, between extreme values of an interference wave with a short period, the period of the trench interference wave is regarded as being fixed, and hence the calculated periods of the trench interference wave (the etching rates of the to-be-etched layer 130) are in tiers as shown in FIG. 24. That is, the resolution is low in the method in which a time period between extreme values is read from a superposed interference wave.
Accordingly, in recent years, a method in which the period of a trench interference wave is calculated by carrying out a frequency analysis without reading a time period between extreme values from a superposed interference wave. In this method, the distribution of frequencies (see FIG. 26A) is obtained from a superposed interference wave by a frequency analysis (for example, a fast Fourier transformation method) to detect the period of a trench interference wave from the distribution of frequencies (see, for example, Japanese Laid-open Patent Publication (Kokai) No. H02-71517).
However, as shown in FIG. 25, there may be the case where a disturbance such as an abnormality in a laser light source or a detector (“I” in the figure) or a disturbance such as an apparent change in period (“II” in the figure) caused by interference of a mask film interference wave and a trench interference wave is added to a superposed interference wave. In the above described method using a frequency analysis, the period of a trench interference wave is only detected from the distribution of frequencies obtained by analyzing a superposed interference wave over the whole time period of etching, and hence if a disturbance is added to the superposed interference wave, the distribution of frequencies is not accurate due to, for example, the occurrence of a peak in an interference period that does not exist under normal conditions (see FIG. 26B), and as a result, the etching amount cannot be stably and accurately calculated.